Last week, Elon Musk-led Tesla introduced its Powerwall home batteries. It was believed that SolarCity customers will be first to get their hands on Powerwall. SolarCity said that Powerwall will replace the “noisy, dirty fossil-fuel generators with zero-emission storage technology.”

SolarCity will offer only 10kWh Powerwall in the beginning

Tesla’s Powerwall will be available in two sizes: 7-kilowatt hours (kWh) and 10kWh. The Palo Alto-based company has designed the 7kWh version for daily use, while its bigger sibling will be used as an occasional backup when the electricity goes out. SolarCity spokesman Jonathan Bass told Tom Randall of Bloomberg that the 10 kWh version can go through only up to 50 charging cycles a year.

SolarCity has decided to offer only the 10 kWh Powerwall, which is designed for daily use, in the beginning. Bass said residential peak shaving “doesn’t really make financial sense” for most U.S. customers because of a policy called net metering. It allows solar customers to sell extra electricity back to the utilities. The San Mateo-based company said it would start offering the 7 kWh option in coming month.

SolarCity’s management system will enable the Powerwall to be configured for a broad range of uses, including battery backup, time-of-use shifting and grid response. The company expects to support any or all of those functions in the future depending on how policy evolves.

You’ll have to wait if you want to use Tesla batteries to move completely off the grid, Bass told Bloomberg. Even if you wanted to use the bigger 10kWh Powerwall, SolarCity requires you to pay $7,140 outright. But with just 2 kilowatts of power, it offers little return on a customer’s investment. Bloomberg notes that 2KW can be easily maxed out by a microwave oven or a hair dryer.

Tesla’s battery a costly affair?

Tesla has designed Powerwall such that you can stack them side by side if you need more power. Unfortunately, SolarCity does not offer any discount on multiple batteries. For instance, if you need 16KW of continuous power, you can buy a $3,700 Generac generator or eight stacked Tesla batteries for $45,000 for a nine-year lease.

Tesla’s Powerwall is a cool technology, but it seems to be a costly affair at this point. Falling battery costs and rapidly evolving electricity regulations may change the scenario in the future, though.

Correction:An earlier version of this article read that Tesla’s 7kWh battery “doesn’t really make financial sense.” SolarCity spokesman Jonathan Bass told us that he was misquoted by Bloomberg, so we ended up using incorrect information. Bass never said that the “7kWh battery doesn’t really make financial sense.” According to his email to us, he said “residential peak shaving doesn’t really make financial sense” due to net metering.